Please refer to FIG. 1 for a conventional technique to detect insulation deterioration of an ungrounded-type or high-impedance grounded-type DC power supply system 9. It includes a grounding alarm device 92 on a power source 91 to measure voltage alterations of the positive terminal and negative terminal of the power supply system 9 respectively against the ground terminal to determine whether circuits of total power supply system 9 have abnormal conditions of insulation deterioration against the ground terminal.
In the event that the power supply system 9 includes a plurality of shunt circuits 93, and one of the shunt circuits 93 has a grounding fault 94 due to insulation deterioration, the grounding alarm device 92 detects the grounding fault 94 and provides an alarm signal by measuring the voltage difference between the positive terminal or the negative terminal of the power supply system 9 and the ground terminal so that the power supply system has to be shut down to inspect insulation of every shunt circuit 93 to identify where the grounding fault 94 occurs. There is no automatic online detection and alarm for the grounding fault 94 of the shunt circuit 93, nor instant cutoff of power supply to the shunt circuit 93 having the grounding fault 94.